Heat the oil and butter in a saucepan, add the onion and fry for 5 minutes until softened but not coloured. Stir in the cauliflower florets and ginger, then the stock. Season with salt and pepper and bring to the boil. Cover and simmer for 15 minutes until the cauliflower is just tender.
Meanwhile, make the glazed seeds by heating the oil in a frying pan, add the seeds and cook for 2–3 minutes, stirring until lightly browned. Add the soy sauce, then quickly cover the pan with a lid until the seeds have stopped popping. Set aside until ready to serve.
Purée the cooked soup in batches in a blender or food processor, then pour back into the saucepan and stir in the milk and half the cream. Bring just to the boil, then taste and adjust the seasoning if needed.
Ladle the soup into shallow bowls, drizzle over the rest of the cream and sprinkle with some of the glazed seeds, serving the remaining seeds in a small bowl for further sprinkling.